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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hi all, I was playing with my half bridge flyback driver, just out of curiosity I put a 50pF cap on the output of my ac flyback and I noticed the arc was about the same length as without the cap, but power draw was less than half. I thought I found a nice way to limit input power while the arc length was nearly the same. I experimented more, I made a cap that read nearly 90pF and connected it to the flyback output. As always I first turned it on with 100W bulb on the mains side for current limiting. However as soon as I started to draw an arc, the bridge shorted out. What I think has happened was resonance on the secondary side of the flyback, but how could this have affected things on the primary side so much that my driver blew in a fraction of second? What's the best way to current-limit a flyback while keeping the same output voltage?
Edit- I've replaced blown parts and removed the HV capacitor, I switch it on, draw an arc, BANG! The 5a fuse remained intact but it tripped the breaker so it seems I have a fault somewhere in the gate drive, it was working great previously, I love this subtle type of faults which to diagnose often costs a lot of silicon lives
EDIT2 - after about 2 hours of messing with my gate drive, I've found I had a blown gate resistor?! What else will I have to see. I'm out of transistors right now (and I will be away for a week or so) so I'll then let you know if it works.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
EDIT2 - after about 2 hours of messing with my gate drive, I've found I had a blown gate resistor?!
Simple problems always have simple causes. The 100pF cap is too small to do anything to the bridge, and IRFP450's already have 400pf output capacitance each.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I don't believe it actually does anything apart from shifting F0 a bit. Did you try to tune the transformer alone? I generally avoid that simply because I prefer lower open-circuit voltage. Longer life for the transformer.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Marko wrote ...
I don't believe it actually does anything apart from shifting F0 a bit. Did you try to tune the transformer alone? I generally avoid that simply because I prefer lower open-circuit voltage. Longer life for the transformer.
Yes I "tuned" it OUT of resonance alone. When I try to tune to resonance, the power draw is enormous, but when messing with it like this I always include the ballast bulb so it limits the current. I don't know why it killed my fets with the cap, could be also due to the defective gate resistor but I don't know. My "goal" is to draw long arcs with the least power possible, and the cap on the secondary seemed a good idea. However it doesn't seem as practical as it appeared to be. As for stressing the transformer, I planned to run two of these flybacks out-of-phase, this way I could easily get 40kv out. I planned to then include the HV cap to decrease power draw and (hopefully) have long (but "thinner") arcs, but I think it is not worth the risk of exploding more silicon.
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Killing my flyback driver
What else will I have to see. I'm out of transistors right now (and I will be away for a week or so) so I'll then let you know if it works. 
